Radar target identification system

ABSTRACT

This system utilizes the low frequency modulations of the signal from a radar which result from target scintillation and target motion. Outputs are provided for three types of identification aural, frequency meter, and automatic recognition computer. A frequency multiplier may be used with the aural and frequency meter outputs.

111 3,733,603 [451 May 15, 1973 RADAR TARGET IDENTIFICATION SYSTEMInventor: Stephen L. Johnston, Huntsville,

Ala.

Assignee: The United States of America as represented by the Secretaryof the Army, Washington, DC.

Filed:

July 31, 1968 Appl. No.: 749,538 1 [56] References Cited UNITED STATESPATENTS 3,478,354 11/1969 Foster et al. ...343/5 R 1,907,471 5/1933Alexanderson.. ..343/5 X 2,659,076 11/1953 Sherr ..343/7.7 3,079,5992/1963 Caspers ....343/7.7 UX 3,360,796 12/1967 Rush ..343/7.7

Primary ExaminerT. H. Tubbesing Attorney-Harry M, Saragovitz, Edvvardd.Kelly,

Herbert Ber] and Aubrey J. Dunn 57 ABSTRACT U.S. Cl. ..343 5 SA 343 7.7

v ThlS system utilizes the low frequency modulations of Int. Cl ..G01s9/02 the si nal from a radar which result from target scintil- Field ofSearch 343/5 R 6 5 R 7 7 lation and target motion. Outputs are providedfor 343/5 three types of identification aural, frequency meter, andautomatic recognition computer. A frequency multiplier may be used withthe aural and frequency meter outputs.

1 Claim, 1 Drawing Figure FREQUENCY METER BAND PASS FREQUENCY FILTERMULTIPUER mwRAJEcToRY) FREQUENCY -l0 4 6 METER I BAND- PASS FREQUENCYFILTER MULTIPLIER mwoDY/OTHER) I AGC/BOXCAR 9 RADAR V'DEO FREQUENCYMETER 5 BAND- PASS (PROPELLER/TURBINE F'LTER w ROTATION) ll 13 h |4AUTOMATIC RECOGNITION COMPUTER D'SPLAY CROSS REFERENCE TO RELATEDAPPLICATIONS An example of the frequency multiplier as mentioned v aboveis the subject of U.S. Pat.-application Ser. No. 702,455, filed Feb. 1,1968 now U.S. Pat. No. 3,487,290. The present system may be used toadvantage with the passive IFF system as disclosed in my U.S. Pat.application, Ser. No. 742,973, filed July 2, 1968 now abandoned.

1 BACKGROUND OF THE INVENTION This invention generally deals with radartarget identification. The commander of an air defense battery must haveknowledge of whether a target is friendly or unfriendly in order todecide whether to attack it. This knowledge is provided by thewell-known IFF. The battery commander would also like to know what kindof target bomber, fighter, helicopter, or missile, and if a missile whatkind? Further, he should know how many targets are present in order toselect the right defensive missile(s) and to optimize firing time(s).Current IFF systems are active, i.e., friendly aircraft carry an activeIFF transponder which responds to an interrogator at the defensivebattery acquisition radar. This concept gives no indication of the typeof enemy target,

- but only that it is an enemy. The air defense battery may also befaced by tactical ballistic missiles, thus requiring more types ofdecisions. IF F is usually impractical for missiles, thus requiring moretypes of decisions. [FF is usually impractical for missiles. A passive,nontransponder-dependent system of target identification is desired.

There are several known passive target identification schemes.Trajectory observation by radar is simple and reasonably accurate. Aheavy bomber does not usually fly at helicopter. speeds and, altitudes.However, a fighter can fly at bomber altitudes and speeds. A mortarround does not fly ICBM trajectories, but some mortar rounds and rocketshave somewhat similar trajectories.

There aie'alsovarious radar signature identifications that may be used;these include those listed below.

1. Propeller/Turbine Modulation 2. High Resolution Radar Cross Section3. Frequency Shift (fine frequency) -4. Target ScintillationPropeller/turbine modulation (1.) is based on the principle that theradar cross section seen by an observing radar contains modulationcomponents due to the rotation of propellers, helicopter rotors,turbines, or

difficult under dynamic conditions, since the aircraft aspect angle maygreatly and rapidly vary.

The frequency shift (fine frequency) (3.), concept is based on a secondmanifestation of the individual scatterers of the high resolutionconcept. If one considers a simple two-body scatter model of scatterersA and B separated by distance L, the resulting radar crosssection is:

o' A B VZAB sin 21rf/c, 2L(t) a change of frequency A f would thus berelated to length. From this aircraftlength can be measured so that aheavy bomber can be differentiated from a fighter.

The equation for the resultant radar cross section 0 the two bodyscatterer model shows a third manifestation. The interference pattern ofthe two scatterers produces a lobe structure, as seen in the typicalradar cross section pattern of an aircraft or missile. A lobe has awidth W of C/( 2jL sin 0) where L sin0 is the projected target length, fis the radar frequency, and C is the propagation constant.

If .this lobe pattern is moved past an observer at an angular rate of0', then a scintillation frequency of WW results. For two scatterersseparated by ft. (i.e., a missile 30 ft. long), at S band with an aspectangular rate of 2/sec., a scintillation frequency of approximately 4 Hzwould result. The scintillation concept is compressors. While this is auseful technique, it does a have problems. Since an aircraft engine doesnot operate at a single constant speed, the problem is complicated. Afurther complication is that somewhat similar or greatly differentaircraft may use the same engine. However, the modulation frequency mayindicate whether the target is a jet aircraft, a propeller aircraft, ora helicopter.

In the high resolution concept (2.), a radar with extremely high rangeresolution is used to observe an aircraft. Resolution sufficient toisolate the radar return from each scatterer on the target must be'used.The engine inlet, radomes, and pilots cockpit will usually be theprinciple scatterers. Interpretation of results can be thus a lengthaspect rate product measurement system.

SUMMARY OF THE INVENTION This invention is a radar target identificationsystem which takes advantage of the low frequency modulation of a radarecho amplitude resulting from target scintillation and motion. Thetargets may be identified by an operator listening to afrequency-multiplied version of the modulation. Also, targets may beidentified by a frequency meter, or by an automatic recognitioncomputer.

BRIEF DESCRIPTION OF THE DRAWING The single drawing FIGURE is a blockdiagram of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Echos from radar targets havebeen found to be modulated in amplitude as the result of three types oftarget motion normal trajectory motion, body motion, and othermotions.Trajectory motion is due to aeroballistic effects and missile attitudeprogramming. Some types of missiles are launched onto a ballistictrajectory at an angle dependent on target range. Other missiles arelaunched vertically or near vertically and are then programmed in pitchangle.

Body motion includes the effects of wind gusts, guidance maneuvers,corkscrewing, nutation, and the like.

Other effects include missile spin, as with the Honest John or Lancemissiles, effects of air brakes as in Sergeant, or configurationchanges, as in Pershing missile separation.

Not all these modulations will be produced by any one missile.Fortunately, the different types of modulations are generally indifferent frequency regions, and are repeatable from one flight toanother.

Recordings of the radar AGC of spinning missiles also contain amodulation which corresponds to a multiple of the rocket spin frequency.

From the study of static radar cross-section patterns, it has been foundthat the lobe width of a particular missile is proportional to thelength of the missile and radar frequency. This lobe pattern is thecause of a trajectory scintillation frequency in a moving missile.Another quantity which determines scintillation frequency fromtrajectory motion is angular aspect rate.

The above described modulations or scintillation of radar signals mustbe properly processed and appropriately utilized to obtain a usefultarget identification system.

The invention may be best understood by reference to the drawing, inwhich reference numeral 1 designates a radar which observes the target.This radar is understood to include conventional transmitting andreceiving circuits, and the usual antenna(s). An output signal from 1 istaken on lead 2. This output may be taken from the radar receiver AGC ormay be a boxcar video. In either event, the scintillation frequencies ofinterest will be in the output. The output signal information on line 2may be utilized in any one of several manners. One manner of use is thatshown at the top of the drawing, wherein the signal from line 2 isapplied to band pass filter 3. Filters 4 and 5 are also connected toline 2. The filters are so constructed that the particular frequenciesof a target may be passed for respective utilization. In particular,filter 3 passes those scintillation frequencies associated withtrajectory motion. The particular frequencies associated with aparticular target may be recognized by a human operator. Thescintillation frequencies are usually subaudible, and it is necessary tofrequency multiply them for the operator. This multiplication may bedone any one of several ways, such as by recording on tape at low speed,and playing at a higher speed. Another frequency multiplication schemeis that shown in my co-pending application having Ser. No. 702,455,filed Feb. 1, 1968. A frequency multiplication is performed in box 6 ofthe drawing, with the output of 6 applied to earphones 7 for anoperator. This output may be also applied to frequency meter 8.

The frequencies for body motion and/or other motion may be treatedsimilar to those for trajectory motion. The output of band-pass filter 4is applied to frequency multiplier 6a, with the output of 7 applied toearphones 9 or frequency meter 10.

The propeller or turbine motion frequencies are high enough that afrequency multiplier is not necessary, and the output of filter 5 isdirectly applied to earphones 11 and/or frequency meter 12.

With the above described circuit arrangement, the identification of aradar target may be made by trained operators using earphones 7, 9 and l1 and/or observing meters 8, l0 and 12.

Targets may also be identified by an automatic recognition computer 13connected to line 2. This computer may give an output to a displaydevice 14. Various types of recognition computers are known which arecapable of target recognition. One type of computer is that described inthe article Adaptive Pattern Recognition and Signal Detection WithoutSupervision by David B. and Paul W. Cooper. This article appeared onpages 246256 of the 1964 IEEE International Convention Record, Part 1.Another example of a computer which may be used in this invention isthat described in the 1965 IEEE International Convention Record, onpages 181-189 of Part 6, Volume 13. Yet another example is found inVolume 72 (Feb. 1965) of American Mathmetics Monthly, on pages 1 11-118.

As mentioned briefly above, the invention may be used for IFF. Afriendly target may carry some means for periodically altering its radarcross-section in some predetermined pattern. The means may consist of arotating corner reflector or Luneberg lens, as taught in my co-pendingapplication having Ser. No. 742,973, filed July 2, 1968. In any event, ascintillation frequency should be generated which is recognizable by anoperator or computer. If desired, one could have ones missile whistleDixie! In this device the multiplication of the frequency multiplier maybe changed if desired to compensate for the effect of the use ofdifferent radar frequency bands in different radars. For example, ifthis device is used with both S band and X band radars for the sametarget types, the frequency multiplier in the device for use with the Sband radar could have a multiplication of four or five times (dependingon the ratio of the two radar frequencies) that of the multiplier forthe X band radar application. This would cause the multipliedscintillation frequencies from the two radars to the nearly equal andthus facilitate operator training.

The outputs from this device can be transmitted over conventional voicefrequency communication channels to permit remote target identificationor comparison of the outputs from two separately located radars whichuse the same or different frequencies.

Other applications of this device are:

1. Counter Mortar/Counter Rocket/Counter Battery In this problem acomputer uses radar derived trajectory data to determine therocket/mortar/missile launch or impact point. It is essential that theproper drag function for the target being observed be used in thetrajectory extrapolation computer. This device will provide properidentification so that an operator or automatic circuitry can select thecorrect drag function based on the target scintillation/body motionfrequencies.

2. Low Altitude Target Tracking In tracking targets in the low altituderegion severe problems can result from the presence of ground clutter.The output signals from this device can be used a means for monitoringthe performance of a radar tracking a low altitude target (eithermanually or automatically) to determine whether the target is stillbeing tracked or is lost in the clutter. Outputs of this device can alsobe used to automatically control the tracking of a target in clutter asa means for discrimination against clutter. This also applies totracking a target in chaff.

3. Decoy/Booster/Reentry Vehicle Discrimination The outputs of thisdevice can be used for discrimination of decoys, booster and reentryvehicles clue to differences of scintillation frequencies and bodymotion frequencies of these objects.

4. Changes of Target Characteristics Outputs of this device can be usedto detect change of the radar signature characteristics of varioustargets such as aircraft, missile rockets. If a target of known type isobserved by radar on a number of occasions and this device is used, anymodification of the target which information on warhead type, burstpattern, effects and the like due to differences in scintillationfrequencies at the time of warhead event. 8. Decoy/Multiple WarheadEjection The outputs of this device can be used to detect the occurrenceof decoy/multiple warhead ejection from a ballistic missile. Knowledgeof the missile altitude from radar data will give further information inthat a decoy is generally ejected at high altitudes whereas a multiplewarhead would generally occur at a different altitude region.

I claim:

l. A radar target identification system including means for illuminatinga target with radar energy, receiving echos having subaudible frequencycomponents from said target, and providing an output indicative of thevariation of the amplitude of said echos; and recognition means foridentifying said subaudible frequency components connected to andresponsive to said-output wherein said recognition means includes aband-pass filter connected to said output; frequency sensitive means;and frequency multiplier means connected between said filter and saidfrequency sensitive means.

1. A radar target identification system including means for illuminatinga target with radar energy, receiving echos having subaudible frequencycomponents from said target, and providing an output indicative of thevariation of the amplitude of said echos; and recognition meAns foridentifying said subaudible frequency components connected to andresponsive to said output wherein said recognition means includes aband-pass filter connected to said output; frequency sensitive means;and frequency multiplier means connected between said filter and saidfrequency sensitive means.